In order to improve the accuracy of an air-fuel ratio control of an internal combustion engine, as shown in Japanese Patent No. 2684011 (U.S. Pat. No. 5,542,404), JP-A-2005-207405 (U.S. Pat. No. 7,051,725B2), and Japanese Patent No. 3357572 (U.S. Pat. No. 5,947,096), there has been performed a cylinder air-fuel ratio control that performs the cylinder air-fuel ratio estimation of estimating the air-fuel ratio of each cylinder by the use of a model for relating the detection value of one air-fuel sensor disposed in an exhaust confluent portion where exhaust gases from plural cylinders merge with each other (air-fuel ratio of the exhaust mergence portion) to the air-fuel ratio of each cylinder. And it is computed an air-fuel ratio correction quantity for each cylinder so as to reduce variation in the air-fuel ratio of each cylinder between the cylinders based on the estimate result of the cylinder air-fuel ratio estimation and controls the air-fuel ratio of each cylinder (fuel injection quantity) based on the air-fuel ratio correction quantity for each cylinder.
In a control unit described in Japanese Patent No. 2684011, the cylinder abnormality diagnosis is performed for determining whether the air-fuel ratio correction quantity for each cylinder computed based on the estimate result of the cylinder air-fuel ratio estimation is within a specified range. It is determined that an abnormality occurs in a cylinder when an air-fuel ratio correction quantity for the cylinder is beyond the specified range.
In the cylinder air-fuel ratio estimation of estimating the air-fuel ratio of each cylinder based on the detection value of one air-fuel sensor disposed in an exhaust confluent portion, the estimate accuracy of the cylinder air-fuel ratio estimation is varied according to the operating range of the internal combustion engine. For example, in a high rotation range where the exhaust interval of exhaust gas of each cylinder is short and in a low load range where an exhaust gas quantity is small, the estimate accuracy of the cylinder air-fuel ratio estimation based on the detection value of the air-fuel ratio sensor tends to decrease.
However, in the control unit described in Japanese Patent 2684011, any consideration is never given to such a change in such estimate accuracy of the cylinder air-fuel ratio estimation that is caused by a difference in the operating range of the internal combustion engine. Accordingly, there is a possibility that the cylinder abnormality diagnosis using the estimate result of the cylinder air-fuel ratio estimation might be performed even within the operating range in which the estimate accuracy of the cylinder air-fuel ratio estimation decreases and that the diagnosis accuracy of the cylinder abnormality diagnosis hence might decrease.
In a control unit described in Japanese Patent 3357572, in order to prevent the estimate accuracy of the cylinder air-fuel ratio from being decreased by the deterioration of the responsivity of the air-fuel ratio sensor, the response speed of an air-fuel ratio sensor is determined and the timing of detecting an air-fuel ratio by the air-fuel ratio sensor is corrected according to the determination result.
When the responsivity of the air-fuel ratio sensor deteriorates, also the output value of the air-fuel ratio sensor may decrease. However, in the control unit described in Japanese Patent No. 3357572, any consideration is never given to the effect of a decrease in the output of the air-fuel ratio sensor when the responsivity of the air-fuel ratio sensor deteriorates. There is a possibility that the estimate accuracy of the cylinder air-fuel ratio based on the output of the air-fuel ratio sensor might be decreased by a decrease in the output of the air-fuel ratio sensor when the responsivity of the air-fuel ratio sensor deteriorates.
In a control unit described in JP-A-2005-207405, any consideration is never given to a change in such estimate accuracy of the cylinder air-fuel ratio estimation that is caused by a change in the operating state of the internal combustion engine. Accordingly, there is a possibility that the estimate accuracy of estimated air-fuel ratio of each cylinder might be decreased by the effect of the operating state of the internal combustion engine, which might involve a decrease in the control accuracy of the cylinder air-fuel ratio control based on the estimated air-fuel ratio of each cylinder.